CN1612793A - Method for laser machining a workpiece with laser spot enlargement - Google Patents

Abstract

An fast steering mirror (30), such as a PMN actuated mirror, is positioned in a beam path (18) of a stage-based positioning system (40) to continuously move a laser beam (46) in a high speed prescribed pattern about a nominal target position (60) to spatially separate focused laser spots (48) generated at a high laser repetition rate and thereby create geometric features having dimensions greater than those of the focused laser spot (48). A series of laser spots (48) at a given repetition rate appear as a series of larger diameter laser spots at a lower pulse rate without beam quality problems associated with working out of focus.

Description

Method for laser processing workpiece by means of laser spot enlargement

technical field

The present invention relates to laser micromachining, and in particular to a method and apparatus for moving a laser spot using a fast deflecting mirror, wherein the laser spot has a focused spot size on a substrate according to a desired pattern for removing the laser spot A target area on a substrate that is larger than the focused spot size.

Background technique

The background art is presented here by way of example only for multi-layer electronic workpieces, such as integrated circuit chip packages, multi-chip modules (MCMs) and high-density interconnect circuit boards, which have become the most preferred in the electronic packaging industry. element.

Devices used to package individual chips, such as ball matrix arrays, pin grid arrays, circuit boards, and hybrid microcircuits, typically include component layers of discrete metals, organic dielectrics, and/or reinforcements, as well as other novel materials. Recently, a great deal of work has been aimed at developing laser-based micromachining techniques for forming vias or other processing in such electronic materials. For micromachining, vias are discussed here by way of example only, and may also take the form of complete vias or incomplete holes known as blind vias. Unfortunately, laser micromachining involves a large number of variables, including laser type, operating costs, and laser- and target-material-specific operating parameters such as beam wavelength, power, and spot size, resulting in wide variations in process throughput and hole quality. big.

For many purposes, pulsed UV lasers currently used in micromachining operations produce small spot sizes compared to the desired kerf width and hole diameter. The throughput of laser machining for forming feature geometries larger than the laser spot size (hereinafter referred to as "topography") can be increased by using larger and less power dense laser beams. Operating the laser out of focus, as described in US Patent No. 5,841,099 to Owen et al., effectively increases the laser spot size and reduces its power density. US Patent Nos. 5,593,606 and 5,841,099, also to Owen et al., describe the advantages of using an ultraviolet laser system to generate laser pulsed output within favorable parameter ranges to form via holes or blind via holes in multilayer devices. These patents refer to well-known techniques by which via holes having a diameter larger than the focused spot size can be produced by perforation, concentric circle processing or spiral processing. These techniques will be referred to as "contoured drilling" hereinafter.

Unfortunately, out-of-focus operation of the laser often results in unpredictable and undesired energy distribution and spot shape and affects via hole quality (including via hole wall taper, degree of melting of copper layer at the bottom of the via hole, and The height of the "edge" surrounding the via hole caused by splashing of molten copper during this period is adversely affected. Furthermore, because the spot size entering conventional collimating and focusing optics is inversely proportional to the spot size striking the target, the power density applied to such optics quickly exceeds the damage threshold of the optics.

US Patent No. 4,461,947 to Ward discloses a method of contour drilling in which a lens is rotated in a plane perpendicular to the incident laser beam to affect a target area that is larger in size than the focused laser spot. The rotation of the lens is independent of the position of the support mounting arm. The Ward patent also discloses a prior art profile drilling method that relies on movement of a mounting arm in a plane to achieve lens rotation. In this background, the Ward patent discloses that a beam of light can be rotated by rotating a mirror.

U.S. Patent No. 5,571,430 to Kawasaki et al. discloses a laser welding system that employs a concave condenser that pivots about a first axis and is supported on bearings by a rotating support member such that the concave condenser can be Rotate about a second axis perpendicular to the first. The concave condenser is rotated about a first axis to increase the "width" of the removed object, and rotated about a second axis to form a ring pattern.

Contents of the invention

It is therefore an object of the present invention to provide a method or apparatus for rapidly spatially distributing a focused laser spot of a high repetition rate pulsed laser and thereby distributing the energy density.

Another object of the invention is to rapidly form geometric features larger than the size of the focused laser spot.

A further object of the present invention is to increase the throughput and/or quality of workpieces in such laser machining operations.

U.S. Patent Nos. 5,751,585 and 5,847,960 to Cutler et al., U.S. Patent No. 6,430,465 B2 to Cutler, contain a description of a split axis positioning system in which the upper table is unsupported by the lower table and is independent of the lower The table moves and in this system the workpiece is carried on one axis or table and the tool is carried on the other axis or table. These positioning systems have one or more upper tables, each of which supports a quick positioner, and are capable of processing one or more workpieces at the same time with high productivity because the independently supported tables carry more load than stacked tables. The system has less inertial mass and is able to accelerate, decelerate or change direction more quickly. Thus, since the mass of one table is not carried by other tables, the resonant frequency for a given load increases. In addition, the slow and fast positioners are adapted to move without stopping in response to the positioning command data stream while coordinating the respective moving positions of the positioners to produce a temporarily fixed tool position at a target position defined by the database. These split-axis, multi-rate positioning systems reduce the travel range limitations of existing systems for rapid positioners while providing significantly improved tooling throughput and the ability to adapt to panelized or unpanelized database to work.

As the overall mass and size of the workpiece increases, while such split-axis positioning systems using longer and thus more massive tables are becoming even more advantageous, they do not provide sufficient bandwidth to efficiently space large geometric intervals The energy is distributed between individual laser pulses at a high pulse repetition frequency (PRF).

The present invention therefore employs a rapidly deflecting mirror, such as a piezo-controlled mirror, to continuously move the laser beam around a nominal target position in a high-speed prescribed pattern in the optical path in order to spatially separate the laser beams generated at high laser repetition rates. of the focused laser spot and thus form geometric features larger than the size of the focused laser spot. The present invention allows a series of laser pulses at a given repetition rate to appear as a series of larger diameter pulses at a lower pulse repetition rate without the beam quality problems associated with out-of-focus operation.

Additional objects and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments with reference to the accompanying drawings.

Description of drawings

1 is a view, partly isometric and partly schematic, of a simplified laser system incorporating fast deflecting mirrors according to the invention.

FIG. 2 is a partly diagrammatic and partly schematic view of a fast deflecting mirror mechanism used in the laser system of FIG. 1 .

3 is a view, partly in section and partly schematic, of a fast deflecting mirror mechanism used in the laser system of FIG. 1 .

Figure 4 is a front view of a fast deflecting mirror demonstrating how bending of the mirror can affect the position of the laser spot.

Fig. 5 is a computer model of an exemplary linear kerf forming profile modified by movement of a fast deflecting mirror in accordance with the present invention.

Figure 6 is a computer model of an exemplary via hole bore profile modified by movement of a fast deflecting mirror in accordance with the present invention.

Detailed ways

Referring to FIG. 1, an exemplary embodiment of a laser system 10 of the present invention includes a Q-switched, diode-pumped (DP) solid-state laser (SS) 12, which preferably contains a solid-state lasing working substance. However, those skilled in the art will appreciate that pump sources other than diodes, such as krypton arc lamps, may also be used. Pump diodes, arc lamps, or other conventional pumping means receive power from a power source (not separately shown), which may form part of the laser 12, or may be provided independently.

An exemplary laser 12 provides a resonantly generated laser output 14 having one or more laser pulses having a predominantly TEM ₀₀ spatial mode distribution. Preferred laser wavelengths are from about 150 nanometers (nanometers) to about 2000 nanometers, including 1.3 _, 1.064, or 1.047, 1.03-1.05, 0.75-0.85 microns (μm) or their second, third, fourth or fifth harmonics, but are not limited to these wavelengths. Such harmonic wavelengths may include, for example, approximately 532 nanometers (frequency doubled Nd:YAG lasers), 355 nanometers (frequency tripled Nd:YAG lasers), 266 nanometers (frequency quadrupled Nd:YAG lasers), or 213 nanometers (frequency frequency-doubled Nd:YAG laser) wavelengths, but is not limited to these wavelengths. Laser 12 and harmonic generation techniques are well known to those skilled in the art. Details of an exemplary laser 12 are described in detail in US Patent No. 5,593,606 to Owen et al. An example of a preferred laser 12 includes the Model 210 UV-3500 laser sold by Lightwave Electronics of Mountain View, California, USA. Those skilled in the art will appreciate that lasers emitting other suitable wavelengths are commercially available and can be used, including fiber lasers, or Q-switched carbon dioxide lasers. US Patent Publication No. US 2002/0185474 Al published December 12, 2002 to Dunsky et al. discloses an exemplary Q-switched carbon dioxide laser.

Referring to Fig. 1, the laser output 14 is subjected to a series of beam orientation elements 20 (such as the positioning mirror of the table axis), a fast deflection mirror FSM (30), and a fast positioner 32 (such as a pair of galvanometers) of the beam positioning system 40. Driven X-axis and Y-axis mirrors) may be processed by a variety of known optical elements, including a beam expander lens element 16 disposed along the optical path 18, before being guided. Finally, the laser output 14 passes through an objective lens 42 , such as a focusing or telecentric scanning lens, before being applied to the workpiece as a laser system output beam 46 having a laser spot 48 .

A preferred beam positioning system 40 is described in detail in U.S. Patent No. 5,751,585 to Cutler et al., and the positioning system may also include the Abbe error (ABBE error) described in U.S. Patent No. 6,430,465 B2 to Cutler et al. ) Calibration device. The beam positioning system 40 preferably employs a mobile stage positioner that preferably controls at least two stages or stages 52 and 54 and supports the positioning element 20 to align and focus the output beam 46 of the laser system to Position 60 on desired laser target. In a preferred embodiment, the moving table positioner is a split axis system in which a Y table 52, typically moved by a linear motor, supports and moves a workpiece 50 along a track 56 and an X table 54 supports and moves along a track 58. By moving the quick positioner 32 and the objective lens 42, the dimension of Z between the X and Y stages is adjustable, and the beam steering element 20 adjusts the optical path by any sub-turn between the laser 12 and the quick deflection mirror 30 18. A typical mobile table positioner is capable of a velocity of 500mm/s and an acceleration of 1.5g. For convenience, the combination of the quick locator 32 and one or more mobile workbenches 52 and/or 54 may be referred to as a main positioning system or an integrated positioning system.

The beam positioning system 40 allows rapid movement between various target locations 60 on the same or different circuit boards or chip packages for single or dual processing operations based on provided test data or design data. An exemplary fast positioner can have a velocity of 400 mm/s or 500 mm/s and an acceleration of 300 g or 500 g, and thus these parameters are also typical performance of an exemplary integrated positioning system. An example of a preferred laser system 10 incorporating many of the positioning system elements described above is Model 5320 Laser System, or other such series, manufactured by Electrical Science Industries, Inc., Portland, Oregon, USA. However, those skilled in the art will appreciate that a system with a single X-Y stage for workpiece positioning, and a fixed beam position and/or quiescent galvanometer for beam positioning can alternatively be used.

Laser system controller 62 preferably synchronizes the activation of laser 12 with the movement of stages 52 and 54 and rapid positioner 32 in a manner well known to those skilled in the art. Laser system controller 62 is shown generally for controlling fast positioner 32 , stages 52 and 54 , laser 12 , and fast deflection mirror controller 64 . Those skilled in the art will understand that laser system controller 62 may include integrated or independent control subsystems to control and/or provide power to any or all of these laser elements, and that such subsystems may be controlled by The arrangement is remote from the laser system controller 62 . The laser controller system 62 also preferably directly or indirectly controls the motion of the fast deflecting mirror 30, including its orientation, inclination or rotation, and speed or frequency, through the mirror controller 64, and controls the components associated with the laser 12 or the positioning system 40. any synchronization. For convenience, the combination of fast deflecting mirror 30 and mirror controller 64 may be referred to as an auxiliary or non-integrated positioning system.

The parameters of the laser system output beam 46 are selected to facilitate adequate clean, continuous drilling, i.e., via hole formation, in a wide variety of metallic, dielectric, and other target materials that may exhibit different Optical absorption, ablation threshold, or other properties. Exemplary parameters of the output of the laser system include an average fluence measured over the spot area of greater than about 120 microjoules (μJ), preferably 200 μJ; less than about 50 microns, and preferably from about 1 micron to 50 microns, typically is a spot diameter or spatial principal optical axis from about 20 microns to 30 microns; a repetition rate greater than about 1 kilohertz (kHz), and preferably greater than about 5 kHz, and most preferably even higher than 20 kHz; wavelength preferably between about 150 nm to 2000 nm, more preferably between about 190 to 1325 nm, most preferably between about 266 nm and 532 nm. The preferred parameters of the laser system output beam 46 are selected in an attempt to avoid certain thermally damaging effects due to the use of instantaneous pulse widths of less than about 100 nanoseconds (ns), and preferably from about 0.1 picoseconds (ns). seconds (ps) to 100 nanoseconds, and more preferably from about 1 to 90 nanoseconds or less. Those skilled in the art will appreciate that these parameters will vary and can be optimized depending on the material being processed, and that different parameters can be used to process different target layers.

The output beam 46 of the laser system preferably produces a spot region 48 having a diameter of less than about 25 to 50 microns at a beam location 60 on the workpiece 50 . Although spot area 48 and diameter generally refer to the 1/ ^e2 dimension, particularly in connection with the description of laser system 10, these terms are occasionally used to refer to the spot area or diameter of the hole formed by a single pulse . Those skilled in the art will also appreciate that the spot area 48 of the output beam 46 is generally circular, but could also be shaped generally square. Those skilled in the art will also appreciate that the output beam 46 can be imaged, or have its wings and tail truncated, if desired for a particular operation, particularly for a first step process.

FIG. 2 shows a preferred embodiment of a fast deflection mirror (FSM) 30 positioned to receive the laser output 14, deflect it through the fast positioner 32, and through the objective lens 42 to a target location on the workpiece 50. 60 for ECB via hole drilling, circuit component trimming, or other micromachining applications. The fast deflecting mirror 30 is preferably implemented as part of a limited deflection beam positioning stage using an electrostrictive actuator with a higher frequency response than the fast positioner 32 . The fast deflecting mirror 30 is deflected by a ferroelectric ceramic actuator material, such as lead magnesium niobate (PMN), and an actuator 22 converts a voltage into a displacement. Lead-magnesium niobate (PMN) materials are similar to more common piezoelectric regulator materials, but exhibit less than one percent hysteresis, high electromechanical conversion efficiency, exhibit wide operating and processing temperature ranges, and do not require permanent polarization, and provide useful mechanical effectiveness with small electrical drive voltages.

The exemplary PMN adjuster 22 has a limited displacement of approximately 20 microns for a 40 mm long PMN cylinder, but a very high stiffness of approximately 210 Newtons per micron for a 5 mm diameter cylinder. The fast deflecting mirror 30 is connected by bends to three PMN regulators 22 having first ends arranged in an equilateral triangle with the center aligned with the center 24 of the fast deflecting mirror 120 . The second end of the PMN adjuster 22 is mechanically connected to the mount 26 attached to the X-axis moving table 54 . The three PMN adjusters 22 are preferably implemented in a three degree of freedom configuration that is used to tilt and flip the fast deflecting mirror 30 in a two degree of freedom mode. The three PMN regulators 22 preferably form a hollow cylinder of PMN material which is electrically divided circumferentially into three active regions. Activating a region causes it to expand or contract, thereby flipping or tilting the fast deflecting mirror 30 .

It is preferred that the adjuster triangle has 5 mm sides so that the fast deflecting mirror 30 can be deflected by an angle of approximately ±4 milliradians ("mRad") when projected onto the workpiece 50 using the 80 mm objective 42. time, it is converted into a deflection of the laser output 14 of ±640 microns. An exemplary fast-deflecting mirror 30 can provide a typical travel-limited range that limits pattern sizes up to about 25 or 50 times the laser spot size; however, the maximum response frequency of the fast-deflecting mirror 30 can be a more constrained , which limits pattern size up to about 15 times the laser spot size, and typically to 5 to 10 times the laser spot size. The fast deflecting mirror 30 operates at a higher frequency and acceleration than the exemplary galvanometer driven X-axis and Y-axis mirrors of the fast positioner 32 . Exemplary fast deflecting mirrors 30 for non-integrated positioning systems provide speeds greater than 1000 mm/s, and can provide speeds of 4000 mm/s or higher, which is 5 to 10 times the speed of typical integrated positioning systems. An exemplary fast deflecting mirror 30 of a non-integrated positioning system provides accelerations greater than 1000 g, and can provide accelerations of 30,000 g or more, which is 50 to 100 times that of a typical integrated positioning system.

In detail, the exemplary PMN regulator 22 has a capacitance of approximately 20 microfarads, a DC impedance of 1.0 ohms, 17 ohms at 5kHZ, and flows over 3 amps when driven at 75V. An exemplary PMN modulator 22 driving a fast deflecting mirror 30 has a large signal bandwidth in excess of about 5 kHz, a small signal bandwidth in excess of about 8 kHz, and at least about 4 Hz for deflecting the laser output 14 with a positioning resolution of ±0.5 microns. Deflection angle in milliradians.

Those skilled in the art will appreciate that any other precision high bandwidth regulator can be used for mirror regulator 22 . FIG. 3 is a partly sectional and partly schematic view of an optional fast deflecting mirror 30 and some exemplary control circuitry 70 of an exemplary mirror controller 64 that is For mirror adjusters 72A and 72B (collectively referred to as mirror adjuster 72), they are preferably piezoelectric (PZT) devices that are used to cause A small change results in a small change in the angle of the laser output beam 46 , which in turn causes a small change in the position 60 of the laser spot 48 on the surface of the workpiece 50 . FIG. 4 is a front view of the fast deflecting mirror 30 demonstrating how mirror bending can affect the position 60 of the laser spot 48 .

3 and 4, in an exemplary embodiment using a PZT mirror adjuster 72, one corner of a generally rectangular fast deflecting mirror 30 is secured to a reference structure having a curved portion capable of bending But it cannot be compressed or stretched. The other two corners of the fast deflecting mirror 30 are driven by sinusoidally responsive piezo mirror actuators 72A and 72B, which will cause the beam position of the laser spot 48 (which is superimposed on the position established by the other elements of the beam positioning system 40) A small angle of change in the target position 60 of ) is introduced into the optical path 18.

In a preferred embodiment, sinusoidal sin(a) signal 74 drives piezoelectric mirror actuators 72A and 72B in opposite directions to create an angular change in one direction, while sin(a+90°) signal 76 drives piezomirror actuators 72A and 72B along Mirror actuators 72A and 72B are driven sinusoidally in the same direction, thereby producing an angular change of 90 degrees to the first angular change described above. The laser output 14 is reflected off the fast deflecting mirror 30 at a point near the center. This results in a circular motion on the workpiece surface after the small angle introduced by the movement of the mirror is translated by the scan lens 42 into a change in position.

For laser drilling operations, the preferred objective focal length is approximately 50-100 mm, the preferred distance from the fast deflecting mirror 30 to the scan lens 42 is as small as possible within design constraints, and when the Z stage (not shown) is preferably less than about 300 millimeters, more preferably less than 100 millimeters at its standard focus height. In the preferred laser system 10, the fast deflecting mirror 30 is mounted upstream of the fast positioner 32 on the X-stage 54 and replaces the final turn mirror in some conventional beam positioning systems. In the preferred embodiment, the fast deflecting mirror 30 is suitable for easily upgrading existing laser and positioning systems 40, such as those employed in the Model 5200 or Model 5300 manufactured by Electrical Science Industries, Inc., Portland, OR, USA, And it can be easily exchanged with the last stage steering mirror on the X workbench 54 of the conventional laser system. Those skilled in the art will understand that, besides the X stage 54 , the fast deflecting mirror 30 can also be arranged at other positions in the optical path 18 .

Those skilled in the art will appreciate that a variety of techniques may alternatively be employed to control the movement of fast deflecting mirror 30 in two axes about a pivot point such as center 24 . These techniques include fast deflecting mirrors 30 using bending mechanisms and voice coil actuators, piezoelectric actuators (which rely on piezoelectric deformation, electrostrictive, or PMN materials), and deforming the mirror's surface electrostrictive or piezoelectric regulators. An exemplary voice coil modulated fast deflecting mirror 30 is described in US Patent No. 5,946,152 to Baker and is suitable for operation at high frequencies. Suitable voice coil actuated fast deflecting mirrors 30 are commercially available from Ball Aerospace Corporation, Broomfield, CO and Newport Corporation, Irvine, CA. A suitable piezoelectric actuator is the S-330 ultra-fast piezoelectric tilt/tilt stage manufactured by Physik Instrumente ("PI") GmbH & Co., Karlsruhe, Germany.

In applications that simulate laser spot magnification, the laser controller 64 commands the stages 52 and 54 of the integrated positioning system and the quick positioner 32 to follow a predetermined tool path, such as a trim profile or a drill profile for a blind via hole, while the mirror The controller 64 independently causes the fast deflecting mirror 30 to move the laser spot position of the laser system output beam 46 in a desired pattern such as small circles or vibrations. This superimposed, free-moving beam movement or vibration distributes the energy of the laser system output beam 46 over a larger area and effectively creates a wider kerf along the tool path. The effective kerf width is usually equal to the pattern size plus the spot diameter. Beam movement also spreads the laser energy over a larger area, effectively increasing the area that can be treated with a given average energy over a period of time.

Because the command sent to the fast deflecting mirror 30 by the mirror controller 64 is not integrated with the positioning commands sent to the workbenches 52 and 54 of the integrated positioning system and the fast positioner 32, but superimposed with it, it can Gain dramatically increased functionality and productivity without a lot of complexity and cost. However, mirror controller 64 may cooperate with laser controller 62 to achieve a particular desired movement pattern of laser system output beam 46 in a particular laser application or in a particular toolpath of the integrated positioning system. The effective spot pattern of the fast deflecting mirror can be selected to have a certain pattern size to obtain a specific kerf width, e.g. hole edge quality. However, those skilled in the art will appreciate that the mirror controller 64 can be programmed directly by the user and does not need to cooperate with or be controlled by the laser controller 64 .

A computer graphics model has been developed to show the individual positions of the laser spot 48 on the workpiece surface resulting from the continuous movement of the fast deflecting mirror 30 by the piezoelectric actuator as described above. FIG. 5B is a computer model of the exemplary linear kerf forming toolpath 80 of FIG. 5A modified for the movement of the fast deflecting mirror 30 . Referring to Figure 5A and Figure 5B (collectively referred to as Figure 5), the parameters include: a pulse repetition frequency (PRF) of about 18 kHz; a spot size of about 25 μm; a linear velocity of about 50 mm/s (a small rotating circular pattern moves through speed of the workpiece surface); rotation rate of about 2 kHz (rate of rotation of the circular pattern); rotation performance of about 30 μm (diameter of the circular pattern (to the center of the beam)); inner diameter of about 10 μm (starting of the spiral pattern diameter (to the center of the circular pattern)); an outer diameter of about 150 μm (the ending diameter of the helical pattern (to the center of the circular pattern)); the number of circles is approximately 2 (the number of rotations of the helical pattern). The model shows that to support laser pulse frequencies in the 15 to 20 kHz range, a rotation rate of 1 kHz to 2.5 kHz (5 to 15 pulses per rotation) is required for practical pulse overlapping.

Referring again to FIG. 5 , the mirror-enhanced linear profile 82 forms a kerf width 84 that is wider than the spot diameter 86 of the output beam 46 . This technique allows fewer cuts to be made wider than the spot diameter 86 while maintaining process quality and other advantages of using a focused output beam 46 (ie, not using a defocused beam to obtain a wider spot). Furthermore, for high repetition rate applications, the mirror-enhanced linear profile 82 can exceed the bandwidth capabilities of most fast positioners 32 and allow fast positioners 32 to retain simple positioning move commands, as opposed to , the sprite additionally requires these positioning move instructions to enable it to implement the sprite within the mirror-enhanced rectilinear shape 82 .

FIG. 6B is a computer model of an exemplary via hole forming rotary toolpath 90 ( FIG. 6A ) augmented by the movement of the fast deflecting mirror 30 . Referring to Figures 6A and 6B (collectively referred to as Figure 6), the parameters include: a pulse repetition frequency (PRF) of about 15 kHz; a spot size of about 15 μm; a linear velocity of about 30 mm/s (a small rotating circular pattern moves through The speed of the workpiece surface); about 1.5kHz rotation rate (rotation rate of circular pattern); about 20μm rotation performance (diameter of circular pattern (to the center of beam)); about 10μm inner diameter (starting point of spiral pattern Starting diameter (to the center of the circular pattern)); an outer diameter of about 150 μm (the ending diameter of the spiral pattern (to the center of the circular pattern)); the number of circles is approximately 2 (the number of rotations of the spiral pattern). The model shows that to support laser pulse frequencies in the range of 15 to 20 kHz, a rotation rate of 1 kHz to 2.5 kHz (5 to 15 pulses per rotation) is required for practical pulse overlapping.

In an exemplary embodiment of a Q-switched CO2 laser system 10 and PMN fast deflecting mirror 30, the CO2 laser system 10 uses a pulse repetition rate of 30 kHz to 40 kHz with 20 to 30 pulses per via hole. The fast deflecting mirror 30 oscillates the laser system output beam 46 at a frequency of 1.0 to 1.5 kHz to make a complete rotation while drilling in less than 0.6 to 1 millisecond.

Referring to FIG. 6, a blind via hole is formed at overlapping adjacent locations by sequentially directing laser system output beam 46 having spot area 86 onto the perimeter along a helical toolpath 90. Referring to FIG. The beam 46 preferably moves continuously through each location at a speed sufficient for the system 10 to provide the necessary number of beam pulses to achieve the depth of cut at that location. As the beam 46 travels along the helical tool path 90, each time the beam 46 is moved to a new cutting position, the target material is "bit by bit" forming a hole of increasing size. The final shape of the hole is typically obtained as the beam 46 travels along a circular path to the perimeter.

Those skilled in the art will note that the mirror-enhanced via hole profile 92 creates a kerf width 84 that is larger than the spot diameter 86 of the output beam 46, so that the resulting via hole diameter 94 is much larger than the kerf. The diameter formed by a helix with the same width as the spot size. The present invention allows a series of pulsed spots 48 with a given repetition rate to appear as a series of larger diameter laser pulsed spots with a lower pulse repetition rate, so that there are no beam quality problems associated with defocusing. Via hole diameters or kerf widths typically range from 25 to 300 microns, although via holes or kerfs with diameters or widths of 1 millimeter (mm) or greater may also be required.

An alternative toolpath for forming blind via holes may start at the center and cut concentric circles of increasing radius defined by kerf width 84 . As the concentric circles forming the vias move on a circular path at greater distances from the center of the zone, the overall diameter of the vias increases. Alternatively, the process can start by defining the desired circumference and work the edges towards the center. The outward spiral machining would be slightly more continuous and faster than the concentric circle machining; however blind via holes could also be formed in an inward spiral.

Those skilled in the art will appreciate that either the workpiece 50 or the process output beam 46 may be fixed or moved relative to the position of the other. In a preferred embodiment, both the workpiece 50 and the process output beam 46 are moved simultaneously. Several examples of through vias and blind vias of different depths and diameters formed over a variety of different substrates are presented in US Patent No. 5,593,606. Various via hole machining techniques, including other tool path profiles, are also disclosed in US Patent No. 6,407,363 B2 to Dunsky et al., which is incorporated herein by reference. Those skilled in the art will understand that non-circular via holes can also be ablated by similar processing. For example, such via holes may have a square, rectangular, oval, slit, or other surface shape.

Those skilled in the art will also understand that the integrated positioning system can be directed to a single location to machine a small area via hole, and the non-integrated fast deflecting mirror 30 is used to form a spot diameter 48 larger in diameter than the output beam 46 access holes without significant action time and without the complexity of moving an integrated positioning system to form a toolpath like toolpath 90. In addition, via hole quality, including edge quality and bottom uniformity, can be greatly improved, especially when the laser system output beam is relatively Gaussian.

It will be apparent to those skilled in the art that many changes can be made to the details of the above embodiments without departing from the principles of the present invention. The scope of the invention should therefore be limited only by the appended claims.

Claims (22)

1. A method of laser processing an effective kerf width on a workpiece with laser output pulses, the laser spot diameter of each of the laser pulses on the workpiece is less than the effective kerf width, and the method includes : master relative motion of a laser spot position is imparted to said workpiece at a first set of defined velocities and accelerations from a master beam positioning system that sets said laser beam from a laser onto said workpiece a beam positioning path for spot positions, said main relative movement defining a main machining path; imparting auxiliary relative motion of the laser spot position to the workpiece at a second set of velocities and accelerations substantially higher than the first set of velocities and accelerations from an auxiliary beam positioning system positioned along the beam positioning path , the auxiliary relative movement is superimposed on the main relative movement and includes a pattern perpendicular to the main processing path with a pattern size less than or equal to about 15 times the laser spot diameter, the The main relative movement and the auxiliary relative movement cooperate to provide the effective kerf width along the main processing path, and the effective kerf width is approximately equal to the pattern size plus the spot diameter. 2. The method of claim 1, wherein the second set includes velocities greater than 1000 mm/s and accelerations greater than 1000 g. 3. The method according to claim 2, wherein said second set comprises velocities from 1000 mm/s to 4000 mm/s and accelerations from 1000 g to 30000 g. 4. The method of claim 1, wherein the first set includes velocities less than 500 mm/sec and accelerations less than 500 g. 5. The method of claim 1, wherein said pattern size is less than or equal to about 10 times said laser spot diameter. 6. The method of claim 1, wherein the assisted positioning system has a large signal bandwidth greater than about 5 kHz and a small signal bandwidth greater than about 8 kHz. 7. The method of claim 1, wherein said second beam positioning system comprises a fast deflecting mirror. 8. The method of claim 7, wherein the fast deflecting mirror comprises a PMN or PZT regulated mirror. 9. The method of claim 1, wherein said main beam positioning system comprises at least one mobile and rapid positioner. 10. The method of claim 9, wherein said rapid positioner comprises at least one galvanometer driven mirror. 11. The method according to claim 9, wherein said quick locator is mounted on a mobile workbench. 12. The method of claim 11, wherein said primary positioning system comprises a split shaft positioning system. 13. The method of claim 1, further comprising performing a via hole drilling operation with the laser output pulses. 14. The method of claim 1, further comprising performing a laser trimming operation with the laser output pulses. 15. A method of using laser output pulses to laser process an effective kerf width on a workpiece, the laser spot diameter of each of the laser pulses on the workpiece is smaller than the effective kerf width, the method comprising : From a moving table positioning system, transferring a laser spot position relative to the table to said workpiece at a speed and acceleration defined by the moving table; transferring rapid relative motion of said laser spot position to said workpiece at a rapidly defined velocity and acceleration from a rapid positioning system having a higher acceleration capability than said mobile table positioning system; In combination with the mobile table positioning system and the fast positioning system, the main relative motion of the laser spot is transmitted to the workpiece at a first set of defined speed and acceleration, and the main beam positioning system is set from the laser to the a beam positioning path of the laser spot position on the workpiece, and the main relative movement defines a main processing path; imparting secondary relative motion of the laser spot position to the workpiece at a second set of velocities and accelerations substantially higher than the first set of velocities and accelerations from a rapidly deflecting mirror positioned along the beam positioning path , the secondary relative motion is superimposed on, but not integrated with, the primary relative motion and includes a pattern perpendicular to the primary processing path having a pattern size equal to or less than about 15 times all The laser spot diameter, the main relative movement and the auxiliary relative movement cooperate to provide the effective kerf width along the main processing path, and the effective kerf width is roughly equal to the pattern size plus the spot diameter. 16. The method of claim 15, wherein the second set includes velocities greater than 1000 mm/s and accelerations greater than 1000 g. 17. The method of claim 16, wherein the second set includes a velocity from 1000 mm/s to 4000 mm/s and an acceleration from 1000 g to 30000 g. 18. The method of claim 15, wherein the rapidly defined velocities and accelerations include velocities of less than 500 mm/s and accelerations of less than 500 g. 19. The method of claim 15, wherein said fast deflecting mirror comprises a PMN or PZT regulated mirror. 20. The method of claim 15, wherein said rapid locator comprises at least a galvanometer driven mirror. 21. The method of claim 15, wherein said primary positioning system comprises a split shaft positioning system. 22. The method of claim 15, further comprising performing a via hole drilling operation with laser output pulses.

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